Bottom Bracket Service: Adjustable Type
This article will discuss the service of adjusting bottom bracket bearings. It will also review the types of tools required.
The basic concept for bottom bracket bearing adjustments is to get the bearings adjusted as loose as possible but without play. This can be done by purposely beginning with play in the adjustment, and then tightening in small increments until play is gone. With much use, the bottom bracket bearings, cups and cones may become worn and pitted. In this case, a smooth bearing adjustment will not be possible. The bottom bracket will need a complete overhaul and most likely new parts as well.
Bearing surfaces are made from hardened steel. The surfaces are cut typically by grinding. Round ball bearings roll on the curved surface of the cup and cone. Even the highest quality bearing surfaces will have slight grinding marks. In the left image below is a high quality cone magnified two hundred times. Notice the parallel marks from the grinding stone. Also note a slight pit from wear. The right hand image is a bearing magnified the same amount. It does show some surface marking, but is generally smoother than the cone or cup. Bearing surface smoothness will vary between manufacturers and between models. Some bearing system will simply “feel” smoother because they are smoother. This is why it is difficult to adjust by using a subjective feeling of smoothness. Generally, adjust bearings for the loosest setting that has no knocking or play, regardless of this relative smoothness.
There is a fairly simple test to detect a worn out bottom bracket bearing. Shift the chain to the inner most rear sprockets front and rear. Drop the chain off the smallest front ring, and arrange it so it will not strike the chainrings. Spin the crank while holding the frame. If you feel an obvious rumbling and/or grinding sensation resonating through the frame, the bearing surfaces are most likely worn out. Internal parts may be required. Typically, the spindle will wear out before the cups. The ball bearings tend to wear out last.
The adjustment procedures below assume the bottom bracket axle, cups and bearings are not badly worn. If a good adjustment cannot be found, it is likely the parts are wore out and need replacement. Trace the bearing path with a ball point pen.
When adjusting the bottom bracket, work in small increments. To help in this, begin by taking about two inches of tape and making marks on one edge every 1/8”. Stick the tape on the left side of the bottom bracket shell so the marks face outward. These will be reference marks when adjusting the bearings and represent the small increments used when turning the adjustable cup.
Adjustable bottom bracket consist an axle and two cups, one on either side of the bike. The drive-side cup (right side) is sometimes referred to as the “fixed” cup because it is secured into the frame. Adjustments are made to the non-drive (left side) cup. If you did not assemble the bottom bracket yourself, it is recommended you check the security of the drive-side cup. A drive-side cup will not allow a proper adjustment. When overhauling and regreasing the bottom bracket, the drive-side cup can remain in place.
NOTE: If bottom bracket bearing surfaces are worn out, it will not be possible to have a smooth adjustment when play disappears. The bottom bracket parts will need replacement.
Shimano® Dura-Ace® and XTR®
These bottom brackets use a cup and cone type system. The bearing adjustment is from the left side, similar to other adjustable bottom brackets. The bottom bracket uses both a ball bearing and a needle bearing system on left and right side cups. When overhauling, it is recommended to leave the needle bearing and ball bearings in the cup. Clean by flushing with solvent.
The non-drive side (left) of the Dura-Ace bottom bracket uses 20 internal splines in the cup. These tend to be shallow. When adjusting the Dura-Ace, use the BBT-22 or BBT-32 to adjust and hold the cup. Secure the lockring with the BBT-7 lockring spanner. Use care to hold the BBT-22 fully into splines of the cup. Note in image below the right crankarm is mounted. Use this as a lever when checking for play.
Adjustable type XTR BB-950 use the BBT-8 for both drive and non-drive cups. The lockring is the BBT-7, as seen in the image below. The procedures for service are similar to the procedure described above.